GENERA AND SPECIES. 85 



rather short, of medium thickness, dark. Apothecia 

 somewhat flattened, dark. Spores rather large, sim- 

 ple, dark-brown; usually spherical; some slightly 

 elongated with pointed tips, exine distinct, 12/x X 20/a. 



Jf.. Acolium. 

 This genus is undoubtedly closely related to Calu 

 cium. The thallus is well developed and of a marked 

 green color in A. tigillare. The stipe is much re- 

 duced, so that the apothecium is sessile or immersed 

 in the thallus. The thallus encloses the apothecium 

 in a crater-like fashion, showing the dark epithecium 

 at the top. The spores are two-celled and dark. 

 The algas are as in CaUcium. 



There are only two species reported in the United 

 States, of which A. tigillare is quite common. Its 

 natural substratum seems to be old pine fence-boards, 

 upon which it occurs in patches of considerable extent. 

 From its position it is evident that it has the ability 

 to resist greater extremes of dryness than the major- 

 ity of lichens. It occurs quite frequently on fence- 

 boards in prairie farm-yards which are comparatively 

 little shaded. 



1. Acolium tigillare. Thallus distinctly crustose 

 and minutely areolate, yellowish-green. Apothecia 

 numerous; the very short stipes, as well as the 

 greater portion of the apothecium, are enclosed by 

 the thallus, producing conical elevations. Epithecium 

 dark. Spores two-celled, somewhat constricted at the 

 middle, thick, dark spore-wall, 10/x X 14/x. 



